The next generation mobile communication system needs to support multiple services such as voice, data, audio, video, image and so on. Therefore, it is desirable for the next generation mobile communication system to support a higher data transmission rate, higher spectrum efficiency and perfect Quality of Service (QoS) guarantee mechanisms, and provide better mobility support and wireless network coverage, so as to provide users with communication services at all times and all places. Presently, the next generation mobile communication system has developed from the second generation mobile communication system and the third generation mobile communication system into the fourth generation mobile communication system. The second generation mobile communication system uses the Time Division Multiple Access (TDMA) and the narrowband Code Division Multiple Access (CDMA) as dominant access techniques, e.g., the Global System for Mobile Communications (GSM) and the CDMA IS-95 mobile communications system. The third generation mobile communication system uses the Wideband CDMA (WCDMA) as dominant access techniques, e.g., the Universal Mobile Telecommunication System (UMTS) and the WCDMA mobile communication system. In the CDMA technique, data symbols of one user will occupy the entire width of carrier frequency and different users or user data are distinguished by means of spread spectrum codes. Since the multi-path channel makes the orthogonality between spread spectrum codes impossible, the CDMA technique becomes a self-interference system. Therefore, the system capacity and spectrum efficiency of the current CDMA technique are unable to meet the requirements of wideband wireless communications.
Since the 1990's, a multi-carrier technique has been in the spotlight among wideband wireless communication techniques. It divides one wideband carrier into multiple sub-carriers on which data are transmitted in parallel. In most system applications, the width of a sub-carrier is less than the coherent bandwidth of the propagation channel. In this way, every sub-carrier demonstrates flat fading in a frequency-selective channel, which makes it possible to reduce inter-symbol interference and may support high-speed data transmission without complex channel equalization required. There are various multi-carrier techniques, for example, the Orthogonal Frequency Division Multiplexing (OFDM), the Multi-Carrier CDMA (MC-CDMA), the Multi-Carrier Direct Spread CDMA (MC-DS-CDMA), the Multi-Tone CDMA (MT-CDMA), the Multi-Carrier TDMA (MC-TDMA), the time-frequency two-dimension spreading technique and other spreading techniques based on the above mentioned techniques.
As a representative technique in multi-carrier techniques, the OFDM technique divides a given channel into multiple orthogonal sub-channels in the frequency domain and permits the overlap of partial frequency spectrum of sub-carriers. As long as the orthogonality between sub-carriers is guaranteed, data signals may be separated from the overlapping sub-carriers.
The OFDM technique was first invented in the middle of the 1960's. The OFDM technique, however, was not widely applied for a long time because the development of the OFDM technique was impeded by many difficulties. Firstly, in the OFDM technique the orthogonality between sub-carriers is required. Although the orthogonality between sub-carriers may be implemented theoretically by means of Fast Fourier Transform (FFT), it is impossible in practical applications to provide a device implementing such complex real time Fourier transform through the technical measures of the day. Secondly, the requirements on the stability of a transmitter oscillator and a receiver oscillator as well as the linearity of a radio frequency power amplifier also prevent the OFDM technique from being applied in practical applications. Since the 1980's, the development of a large scale integrated circuit technique has solved the problem of implementing the FFT. Along with the development of the Data Signal Processor (DSP) technique, the OFDM technique has been turned from the theory into practical application.
The OFDM technique rapidly becomes a study focus due to its inherent strong resistance to delay spread and its high spectrum efficiency, and is adopted by multiple international specifications such as the European Digital Audio Broadcast (DAB), the European Digital Video Broadcast (DVB), the High Performance Local Area Network (HIPERLAN), the Institution of Electrical and Electronics Engineers (IEEE) 802.11 Wireless LAN (WLAN) and the IEEE802.16 wireless Metropolitan Area Network (MAN). The multi-carrier technique was discussed as a dominant access technique at the Radio Access Network (RAN) conference of 3rd Generation Partnership Project (3GPP) held in 2004.
In 1947, the Bell Laboratories set forth the cellular concept. Owing to the propagation fading of radio waves, one carrier frequency may be reused outside a certain distance. Thus, compared with the macro-cell networking technique, the cellular technique has higher spectrum efficiency. FIGS. 1A to 1C are simplified schematic diagrams illustrating the conventional frequency reuse pattern with the reuse factor equal to 2, 3 and 4 respectively.
In the first generation mobile communication system Frequency Division Multiple Access (FDMA) and the second generation mobile communication system TDMA, 7, 9 or 11 is usually chosen as the frequency reuse factor to eliminate the co-channel interference. In the CDMA technique, the frequency reuse factor 1 is adopted, the same carrier frequency is used in all cells, and different scramble codes are set to distinguish one cell from another. In this way, complex frequency planning is not needed, and it is easy to implement soft switching and improve the spectrum efficiency. Therefore, the frequency reuse factor being set as 1 is regarded as a great advantage of the CDMA technique.
The forgoing techniques are frequency-based network planning. Of course, analogously to the frequency reuse scheme, a time reuse scheme may also be used, that is, the time is divided into time slots. The inter-cell interference may be reduced when different cells use different time slots.